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DUBNIUM

IVO J. ZVARA, JOINT INSTITUTE FOR NUCLEAR RESEARCH, DUBNA

In the late 1950s, Georgy Flerov, the discoverer of spontaneous fission of uranium (in 1940) and key "coauthor" of the Soviet atomic bomb, was fully back to basic research. He felt that the interactions of accelerated "heavy ions" (boron, carbon, oxygen, and so on) might open great prospects in nuclear physics and chemistry. At the top of the list would be the production of exotic nuclides. He pushed forward the project for a dedicated 300-cm heavy-ion cyclotron. The machine was built at the international Joint Institute for Nuclear Research (JINR) established in 1956 at Dubna, Moscow. Flerov's international research team consisted mostly of young scientists and engineers. The nuclear sciences then were very prestigious, and such a famous leader could choose from a number of good to excellent graduates applying for the positions. Most of them soon became well-known experts.

Of the broad research program at the new JINR Laboratory of Nuclear Reactions (today named after Flerov), the most ambitious goal was synthesis of new chemical elements, starting with number 102. Such research had been systematically conducted only by Glenn T. Seaborg and his collaborators at the University of California, Berkeley, and resulted in the discovery of elements 94 to 101. With the heavy ions, the simple basic idea is to fuse the projectiles with suitable heavy targets. But even if formed, the compound very seldom survives prompt fission, and the bombardment produces a tremendous amount of radioactive "by-products." With the higher atomic number of the new element, the effective cross-section for its production drops steeply, and it is extremely technically difficult to develop more and more intense accelerator beams and targets that can withstand them. But even more serious problems arise in isolation, measurement of decay properties, and conclusive identification of the atomic and mass numbers of the wanted atoms. One could attempt purely physical methods or combine these with radiochemical isolation and identification; the latter had been the case with the relatively long-lived elements 93 to 101.

At Dubna, from the very beginning, Flerov insisted on the development, however difficult, of new fast radiochemical methods for the transactinides, elements 104 and beyond. Their first isotopes in sight were expected to live seconds or less, and traditional techniques could not cope with such lifetimes and the minute yields.

APPARATUS The chemistry of volatile compounds of elements 104 to 106 (with half-lives of only a few seconds) were studied using this machine.
COURTESY OF IVO J. ZVARA
The young newcomers to the field at Dubna were very enthusiastic; they learned fast and worked hard. The cyclotron U-300 was put in operation in 1960, and just three years later--after development of a number of original techniques, methods, and approaches--Dubna was able to claim production of element 102. During the subsequent decade, the claims could be extended up to element 106 and included radiochemical identification of the first transactinides by a new, fast "gas phase" method.

The research was paralleled by the work at Berkeley with competing claims of discoveries. After years of disputes, the two laboratories failed to agree on priorities and names for the new elements. In 1985, the International Union of Pure & Applied Chemistry (IUPAC) and the International Union of Pure & Applied Physics established the Transfermium Working Group, consisting of renowned impartial nuclear scientists. After five years of work, the experts concluded that element 102 (nobelium) was discovered at Dubna; that element 103 (lawrencium) was the result of combined works of Dubna and Berkeley; that element 104 (rutherfordium) was discovered contemporaneously and independently at Dubna (identification by chemistry) and Berkeley (by physical methods); that element 105 (dubnium) was discovered by physical techniques also contemporaneously and independently in the two laboratories; that element 106 (seaborgium) was discovered at Berkeley; and that the major credit for 107 to 109 (bohrium, hassium, and meitnerium) went to GSI Darmstadt, where research started in the late 1970s.

We were thrilled when IUPAC issued its recommendation to name element 105 dubnium "to recognize the distinguished contributions to chemistry and modern nuclear physics of the international scientific center."

In recent years, the Dubna research team, now headed by Yuri Oganessian, has reported major breakthroughs regarding the long-awaited "island of stability" around atomic number 114, where some of the heavy nuclei might live much longer than isotopes of elements 102 to 108. When bombarding uranium and transuranium (up to californium) targets with extremely intense beams of 48Ca, the researchers discovered several new, relatively long-lived -and spontaneous-fission nuclides that can be assigned to elements 110 to 118! But this subject undoubtedly deserves a separate paper.


Ivo J. Zvara has been with the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna, Russia, since 1960. He was the leader of radiochemical studies of synthetic elements and pioneered gas-phase chemistry of transactinide elements.
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DUBNIUM AT A GLANCE
Name: Named after Dubna, Russia, the site of the Joint Institute for Nuclear Research (JINR).
Atomic mass: (262).
History: A team from JINR first reported producing dubnium in 1967. In 1970, both the Russian team and a team from Lawrence Berkeley National Laboratory confirmed the discovery.
Occurrence: Does not occur naturally.
Behavior: Highly radioactive.

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